My Harrison tank is the highest point as well. It has to be, if the halfway point of the Harrison is same height as the highest part of the cooling system (upper radiator hose). My guess is mine is mounted near same height as yours. The radiator cap is about 1” below that part of the hood.
You commented about, Expelling into top of catch can. That’s just it, if it does expell into the overflow/catch can, I do want it to recover it. Otherwise the level
of coolant in the system just dropped that amount, just like it did years ago. So you can plumb into the catch can so it can recover the expelled amount; but only that amount.
In these posts I feel I’ve answered my own question. The catch can (recovery, in
my case), must not filled with any additional
antifreeze.